Magenta fog makes objects in foreground green tinted?
You're using some tricky compositing nodes here. A lot of compositor workflow uses "blending styles" (see the Mix RGB node dropdown for a complete list). Some styles have a procedural approach that creates extremely strange hue shifts and color distortion if you don't know what you're dealing with. 

Since you're getting an unexpected green cast / hue-shift, it might be the Glare node to blame. It's really not designed for this kind of colored fog. You'd be better off either compositing in a discrete comp. app or using a custom node setup (e.g. Z-depth or fog render layer mask + Mix RGB > Hue). With your current setup, you'll have all sorts of unexpected visual results.